Method for dewatering sludge by combining biological enzyme and deep eutectic solvent

Abstract

The application provides a method for sludge dewatering by combining biological enzyme and deep eutectic solvent, comprising: adding biological enzyme to sludge for hydrolysis to obtain enzyme-hydrolyzed sludge, adding deep eutectic solvent to the enzyme-hydrolyzed sludge for conditioning, and mechanically dewatering the conditioned sludge, wherein the deep eutectic solvent comprises choline chloride and organic acid. The method can shorten the capillary water absorption time and reduce the specific resistance of filter cake by first hydrolyzing to destroy the extracellular polymeric substance (EPS) network of sludge and then using DES for deep modification and hydrophobic reconstruction, and can also significantly reduce the water content of the sludge cake.

Description

Technical Field

[0001] This invention relates to the field of sludge treatment technology, and in particular to a method for sludge dewatering using a combination of biological enzymes and deep eutectic solvents. Background Technology

[0002] Municipal sludge has a high content and stable structure of extracellular polymeric substances (EPS), with water firmly bound in the form of bound water, making mechanical dewatering difficult. Traditional chemical conditioning suffers from problems such as high chemical consumption and secondary pollution; while single biological enzyme treatment can selectively hydrolyze the protein components in EPS and destroy the gel-like network, it is prone to causing solid-liquid separation deterioration due to the exposure of hydrophilic amino acids / peptides produced by enzymatic hydrolysis. This is manifested as an increase in CST (capillary time to water absorption) and SRF (specific resistance), and the moisture content of the sludge cake is usually still around 70%; DES treatment alone is limited by the sludge gel structure, making it difficult to fully contact the internal water-holding matrix, resulting in insufficient depth of action, and large doses of DES will increase the viscosity of the system and increase costs. Summary of the Invention

[0003] The purpose of this invention is to provide a method for the combined treatment of sludge dewatering by biological enzymes and deep eutectic solvents, in order to solve the problems existing in the prior art. This invention first degrades the EPS network by enzymatic hydrolysis, and then performs deep modification and hydrophobic reconstruction using DES, so as to reduce CST and SRF and significantly reduce the moisture content of the sludge cake.

[0004] To achieve the above objectives, the present invention provides the following solution: This invention provides a method for sludge dewatering using a combination of bio-enzymes and deep eutectic solvent (DES). This method employs a step-by-step synergistic conditioning approach to achieve deep dewatering. Addressing the contradiction that enzymatic hydrolysis can destroy EPS but may introduce hydrophilic small molecules leading to a rebound in CST / SRF, and that DES alone is insufficient in its effect, this invention proposes a sequential synergistic strategy: first, enzymatic hydrolysis to open channels, followed by molecular-level hydrophobic modification with DES, to further reduce the moisture content of the sludge cake. The method includes: (1) Enzymatic hydrolysis stage: Bioenzymes are added to the sludge for hydrolysis to obtain enzymatically hydrolyzed sludge; this enzymatic hydrolysis stage can destroy the water-holding network of extracellular polymeric substances (EPS) in the sludge and form interconnected channels. Bioenzymes include neutral proteases, acidic proteases and / or alkaline proteases. The choice of enzyme type is determined by the properties of the sludge. They can selectively hydrolyze the protein components in EPS, weaken the cross-linking points of the protein-polysaccharide complex gel, reduce the binding of the network to bound water and release some water, while loosening the internal matrix and forming diffusion channels that facilitate the subsequent penetration of small molecules.

[0005] (2) Add a deep eutectic solvent (DES) to the enzymatically hydrolyzed sludge for conditioning, and mechanically dewater the conditioned sludge. The deep eutectic solvent DES includes choline chloride and organic acids. During the DES conditioning stage, rapid mixing for 1–5 min followed by slow mixing for 5–25 min can be used to balance penetration and floc reconstruction. DES stage: hydrogen bond competition and dehydration modification induce hydrophobic aggregation and skeletalization of flocs. Taking ChCl-organic acid type DES as an example, it has multi-site hydrogen bond donor / acceptor characteristics, which can compete and recombine with the amino acid residues, hydroxyl / carboxyl groups and other functional groups exposed by enzymatic hydrolysis, partially shielding hydrophilic groups and changing the interfacial hydration layer structure; at the same time, the swelling / dissociation effect of DES on the EPS system can promote the conversion of bound water to free water and make the particle surface tend to be hydrophobic. The above effects together promote hydrophobic aggregation, particle bridging and structural skeletal reconstruction of flocs, reducing compressibility and filtration resistance. The synergistic treatment principle of bio-enzymes and DES can be summarized as: structure opening - site exposure - molecular modification - floc reconstruction. After synergistic treatment, the zeta potential approaches electroneutrality, electrostatic repulsion is weakened, and particles are more likely to approach each other and form larger and denser floc structures, thus making it easier to remove water and obtain lower cake moisture content in subsequent mechanical dewatering.

[0006] Furthermore, this invention has conducted corresponding research on the order of adding DES and bioenzymes, specifically as follows: Since DES (especially those composed of choline chloride and organic acids) is typically acidic, and the activity of bioenzymes (such as neutral proteases) is highly dependent on a specific pH environment, mixing these two will result in direct chemical conflict. Adjusting the order of addition may lead to loss of enzyme activity (if DES is added first or simultaneously, the acidic DES will immediately change the pH of the sludge system, causing it to deviate from the optimal pH range of the bioenzyme (neutral protease). This will lead to enzyme denaturation or inhibition of the active site, thus significantly weakening the ability to hydrolyze EPS. The purpose of enzymatic hydrolysis is to open channels; when enzymatic hydrolysis is not fully carried out, the EPS network structure remains intact, making it difficult for DES molecules to penetrate into the interior of the sludge flocs, and they can only act on the surface. This not only wastes DES but also fails to achieve molecular-level modification and hydrophobic reconstruction of the internal exposed sites.

[0007] Furthermore, even if DES does not completely kill the enzyme, the rapid swelling / dissociation effect of DES on EPS when DES and biological enzymes are added simultaneously may alter the physical structure of the sludge, hindering the binding of the enzyme to specific substrates (proteins) and resulting in low enzymatic hydrolysis efficiency. Therefore, this invention adopts an operational strategy of first opening the channels with enzymatic hydrolysis followed by molecular-level hydrophobic modification with DES.

[0008] Furthermore, in step (1), the dosage of the bio-enzyme is 20–60 mg / g, preferably 30–50 mg / g, based on the total suspended solids in the sludge.

[0009] Furthermore, the bioenzyme includes a neutral protease.

[0010] Furthermore, in step (1), the hydrolysis temperature is 35-45℃ and the hydrolysis time is 1-3 hours.

[0011] Furthermore, the stirring speed is controlled at 80–200 rpm during hydrolysis, preferably 150 rpm.

[0012] Further, in step (2), DES is added to the enzymatically hydrolyzed sludge within 0–30 min after step (1) to inhibit regelation / hydrophilic rebound, ensuring that the exposed hydrophilic sites do not regel and providing channels for DES penetration. Introducing DES within 0–30 min after enzymatic hydrolysis to perform molecular-level modification and hydrophobic reconstruction of the exposed sites can effectively suppress the rebound of CST and SRF that may occur under enzyme treatment alone, achieving simultaneous improvement of solid-liquid separation resistance indicators. In a short period after enzymatic hydrolysis (preferably 0–30 min), there are a large number of exposed hydrophilic functional groups and newly generated small molecule peptides / amino acids in the system. If not adjusted in time, these hydrophilic components may re-adsorb / rearrange and lead to increased colloidal stability, resulting in insignificant improvement or even rebound of CST and SRF. This invention limits the introduction of DES to this window period, allowing its effect on the exposed sites to occur preferentially, reducing the risk of enzymatic hydrolysis adverse reactions from the source.

[0013] Further, in step (2), the amount of DES added is 1-3 mL / 100 mL sludge, preferably 1 mL / 100 mL sludge, based on the sludge volume.

[0014] Furthermore, the molar ratio of choline chloride to organic acid is 1:0.5–1.5, preferably 1:1, and the organic acid is preferably citric acid.

[0015] Furthermore, the mechanical dehydration includes pressure filtration, centrifugation, or vacuum filtration.

[0016] Furthermore, the method for preparing the deep eutectic solvent includes: drying choline chloride and organic acid separately, mixing them in a preset ratio, stirring at 60–100°C until clear, and cooling to obtain the deep eutectic solvent.

[0017] The present invention discloses the following technical effects: This invention provides a method for sludge dewatering using a combination of bio-enzymes and a deep eutectic solvent. This method employs a synergistic strategy of first opening channels through enzymatic hydrolysis, followed by molecular-level modification and hydrophobic reconstruction using DES. Enzymatic hydrolysis weakens the EPS network and forms permeable channels. DES is introduced within 0-30 minutes after enzymatic hydrolysis to perform molecular-level modification and hydrophobic reconstruction on exposed sites. This effectively suppresses the rebound of CST and SRF that may occur under enzyme-only treatment, achieving simultaneous improvement in solid-liquid separation resistance indicators. After synergistic treatment using this method, the sludge zeta potential can be increased from approximately -15.6 mV to near-neutral (approximately 0.43 mV), which helps reduce electrostatic repulsion between particles, promotes floc aggregation and structural framework formation, thereby improving the efficiency of mechanical dewatering such as pressure filtration, centrifugation, or vacuum filtration. The sludge cake moisture content can be reduced from approximately 77.5% to approximately 60.12%. This method is mild and can be directly integrated into existing mechanical dewatering processes, improving dewatering performance without significantly increasing chemical consumption and viscosity burden, facilitating large-scale application. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a comparison chart of cake moisture content (WC) and comprehensive dewatering resistance index (CST, SRF) under different treatment methods of the present invention; Figure 2 This is a comparison chart of water content (WC) and comprehensive dehydration resistance index (CST, SRF) under different addition sequences of DES and bioenzymes in this invention. Figure 3 This is a response surface / contour plot showing the effects of enzyme dosage, hydrolysis time, and DES dosage on the moisture content (WC) of the sludge cake when the present invention uses response surface methodology (RSM) for multi-factor optimization. Figure 4 This is a comparison diagram of sludge particle size distribution and Zeta potential under different treatment methods according to the present invention; Figure 5 This is a schematic diagram illustrating the mechanism of action of the synergistic treatment of DES and biological enzymes in this invention. Detailed Implementation

[0020] Various exemplary embodiments of the present invention are now described in detail. This detailed description should not be considered as a limitation of the invention, but rather as a more detailed description of certain aspects, features, and embodiments of the invention. It should be understood that the terminology used herein is merely for describing particular embodiments and is not intended to limit the invention. Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of the invention. All documents mentioned in this specification are incorporated by reference to disclose and describe methods and / or materials associated with those documents. In the event of any conflict with any incorporated document, the content of this specification shall prevail. Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0021] I. Materials and Methods (1) Sludge: Excess activated sludge / biological sludge from municipal wastewater treatment plants. To reduce particle size distribution and impurity interference, the sludge is preferably filtered through a 40-mesh sieve, and after filtration, it is allowed to settle for about 6 hours and the supernatant is removed, so that the sludge moisture content is about 98%. The treated sludge sample is stored for a short period of time at 4°C, preferably within 5 days.

[0022] (2) Bioenzyme: Neutral protease (commercially available preparation). Since the sludge used in this invention is a neutral sample, neutral protease is preferred.

[0023] (3) Preparation of DES: Choline chloride and citric acid are mixed in a 1:1 molar ratio and heated and stirred at 70–90°C until clear and transparent. The mixture is then cooled for later use. Choline chloride, citric acid and other organic acids are all commercially available.

[0024] (4) Dewatering: Vacuum filtration or plate and frame filter press can be used. The moisture content of the sludge cake is determined by conventional methods. The specific process parameters are as follows: Vacuum filtration: Vacuum pump vacuum pressure -0.1 MPa, filter until no more water droplets fall; Plate and frame filter press: Plate and frame machine pressure 0.2 MPa (air compressor power 1.5 KW), filter press time 5 min; Conventional methods for determining moisture content: Dry the crucible to constant weight and record its mass as m0. Take an appropriate amount of sample sludge cake, place it in the crucible, weigh it, and record its mass as m1. Place it in an oven at 105℃ to dry to constant weight, cool it, weigh it, and record its mass as m2. The sludge moisture content is... The calculation formula is as follows: II. Implementation Examples Example 1 100 mL of pretreated sludge sample was placed in an Erlenmeyer flask. Based on the total suspended solids (TSS) in the sludge, 40 mg / g of neutral protease was added, and the mixture was hydrolyzed at 40°C and 150 rpm for 2 h. Subsequently, 1 mL of DES (choline chloride:citric acid = 1:1) was added over 30 min, and the mixture was further reacted for 20 min. The treated sludge was then vacuum filtered and dewatered (vacuum pressure of the pump was -0.1 MPa) to obtain a sludge cake, as described in Example 1. Figure 1 The NP+DES group in the middle.

[0025] Comparative Example 1 (Enzyme Treatment Only) The neutral protease 40 mg / g TSS was added under the conditions of Example 1, without the addition of DES. The remaining treatment steps were the same as in Example 1. The results are shown in [Figure 1]. Figure 1 In the NP group: the decrease in moisture content of the mud cake was limited, approximately 72.0% ( Figure 1 a) CST and SRF were higher than in control sample 3 (see Figure 1 b).

[0026] Comparative Example 2 (DES processing only) Under the conditions of Example 1, only 1 mL of DES (choline chloride:citric acid = 1:1) was added to the sludge, without adding any biological enzymes. The remaining treatment steps were the same as in Example 1, and the results are shown in [Figure 1]. Figure 1 The medium DES group showed improved dewatering performance, with a cake moisture content of approximately 65.17% (see...). Figure 1 a), but still higher than the synergistic processing effect of Example 1, and CST decreased to 24.5 s, SRF decreased to 12.28×10 13 m / kg (see Figure 1 b).

[0027] Comparative Example 3 (blank control, for) Figure 1 (RS group in the middle) Under the conditions of Example 1, the sludge was directly dewatered by vacuum filtration without any conditioning, and the results are shown in [Figure 1]. Figure 1 Middle RS group: The mud cake has a high moisture content and is difficult to dehydrate; the moisture content of the mud cake is approximately 77.5% (see...). Figure 1 a) CST is 25.4 s, SRF is 15.4 × 10 13 m / kg (see Figure 1 b).

[0028] from Figure 1(a) It can be seen that, compared with Comparative Examples 1-3, the moisture content of the sludge cake in this embodiment of the NP+DES group can be reduced to about 60.12%, while the moisture content of the sludge cake can be reduced to about 60.12%. Figure 1 (b) It can be seen that, compared with comparative examples 1-3, the CST of the NP+DES group decreased from 25.4 s to 19.2 s, and the SRF decreased from 15.4 × 10⁻⁶. 13 m / kg decreased to 9.55×10 13 m / kg (see) Figure 1 (b) The significant reduction in CST and SRF indicates that following this synergistic conditioning sequence can effectively overcome the rebound in dehydration resistance caused by enzyme treatment alone.

[0029] In addition, the particle characteristics and zeta potential changes of Examples 1 and Comparative Examples 1-3 are shown in [reference needed]. Figure 4 .from Figure 4 It can be seen that the zeta potential of the sludge after co-treatment can be increased from approximately -15.6 mV to near electroneutrality (approximately 0.43 mV) (see...). Figure 4 (b) This synergistic treatment helps reduce electrostatic repulsion between particles, promotes floc aggregation and structural framework formation, facilitates floc aggregation and solid-liquid separation, and the experiment also observed that the flocs transformed from a hydrophilic gel state to a porous framework state, thereby promoting water migration. Furthermore, compared with the RS treatment, the D50 decreased from 76.9 μm to 64.9 μm (see [link to treatment]). Figure 4 a) This indicates that after co-treatment, the sludge flocs underwent EPS deconstruction and structural rearrangement at the microscopic level, manifested as optimized particle size distribution and improved surface properties. Although the median particle size D50 decreased, the particle surface charge tended to be neutral, compressibility decreased, and internal drainage channels became more unobstructed, thus improving the overall mechanical dewatering performance. The NP treatment group showed increased particle size, the DES treatment group showed decreased particle size, while the NP+DES combined treatment showed synergistic optimization of particle structure and surface charge, which is more conducive to subsequent mechanical dewatering. Figure 5 This diagram illustrates the mechanism of action of DES and bioenzyme synergistic treatment. Specifically, NP and DES conditioning significantly improve sludge dewatering performance through molecular-macroscopic multi-scale synergistic effects, with the hydrogen bonding of DES playing a decisive role in the entire modification process. Firstly, NP specifically cleaves protein components in EPS, effectively disrupting the three-dimensional protein network framework and converting some bound water into free water, creating favorable conditions for subsequent deep modification by DES. Subsequently, DES, with its unique hydrogen bond acceptor and donor properties, further promotes water release.

[0030] Simultaneously, it forms stable hydrogen-bonded complexes with the amino acid residues exposed after enzymatic hydrolysis, reconstructing the surface hydrogen bond network. Under the action of DES, the proportion of α-helices decreases and the proportion of β-sheets increases. This hydrogen-bonded effect induces protein chain rearrangement and the formation of new aggregate structures, weakening the binding ability of EPS to water molecules and promoting the release of bound water. Furthermore, after the EPS barrier is broken down, the hydrogen bonding of DES further acts on the microbial cell membrane, causing cell structure destruction and releasing intracellular water trapped inside the cell, achieving a deep release from extracellular bound water to intracellular water. This multi-scale synergistic mechanism with hydrogen bonding as its core fundamentally changes the physicochemical properties of sludge, thereby improving sludge dewatering performance.

[0031] Comparative Example 4 (DES first, then bioenzyme) Take 100 mL of pretreated sludge sample and place it in an Erlenmeyer flask. Add 1 mL of DES (choline chloride:citric acid = 1:1) based on the total suspended solids (TSS) in the sludge and mix and react for 20 min. Then add 40 mg / g TSS of neutral protease within 0–30 min and hydrolyze the sample at 40℃ and 150 rpm for 2 h. The treated sludge is then dewatered by vacuum filtration (vacuum pressure of water pump (-0.1 MPa)) to obtain sludge cake. Figure 2 (a) According to the DES+NP group, compared to Example 1, the moisture content of the mud cake in this comparative example can be reduced to about 70%, while... Figure 2 (b) According to the DES+NP group, compared with the original sludge, CST only decreased from 25.4 s to 23 s, and SRF only decreased from 15.4 × 10⁻⁶. 13 m / kg decreased to 13.6×10 13 The m / kg indicates that acidic DES immediately alters the pH of the sludge system, causing it to deviate from the optimal pH range for the biological enzymes (neutral proteases). This leads to enzyme denaturation or inhibition of active sites, significantly weakening the ability to hydrolyze EPS. Since this enzymatic hydrolysis step does not occur, the goal of opening channels cannot be achieved, and therefore the moisture content does not decrease significantly.

[0032] Comparative Example 5 (with DES and bio-enzymes added simultaneously) Take 100 mL of pretreated sludge sample and place it in an Erlenmeyer flask. Add 1 mL of DES (choline chloride:citric acid = 1:1) and 40 mg / g neutral protease TSS according to the total suspended solids (TSS) in the sludge. React at 40℃ and 150 rpm for 2 h. Then, the treated sludge is vacuum filtered and dewatered (vacuum pressure of water pump (-0.1 MPa)) to obtain sludge cake. Figure 2(a) With the simultaneous addition of DES and neutral protease (DES-NP), the moisture content of the sludge cake in this example increased to 79.5% compared to Example 1, while... Figure 2 (b) According to the DES-NP group, compared with the original sludge, CST increased from 25.4 s to 26.5 s, and SRF also increased from 15.4 × 10⁻⁶. 13 m / kg increased to 15.9×10 13 The m / kg indicates that when DES and NP are added simultaneously, DES does not completely kill the enzyme, and the rapid swelling / dissociation effect of DES on EPS may change the physical structure of the sludge, which in turn hinders the binding of the enzyme to specific substrates (proteins), resulting in low enzymatic hydrolysis efficiency and even deterioration of sludge dewatering performance.

[0033] Example 2 (Increasing Enzyme Amount) The sludge was treated according to the method in Example 1, except that the neutral protease dosage was 50 mg / g TSS, while the remaining treatment steps were the same as in Example 1. The results are shown in Table 1: Under the conditions of 50 mg / g TSS enzyme dosage, 2 h hydrolysis time, and 1 mL DES dosage, the sludge cake moisture content could be reduced to approximately 66.94%, still better than the blank control. The sludge cake moisture content in this example is slightly higher than in Example 1. This may be because in this example, the neutral protease dosage was 50 mg / g TSS. Under this dosage, a large amount of protein in the sludge would be hydrolyzed into hydrophilic small molecules. Although the subsequent DES would convert some of the hydrophilic substances into hydrophobic substances, the remaining hydrophilic substances would worsen the sludge dewatering performance, preventing the complete release of trapped water. Therefore, the moisture content was actually higher than that of the sludge treated with DES alone.

[0034] Example 3 (Adjusting the hydrolysis time) The sludge was treated according to the method in Example 1, with the difference that the neutral protease dosage was 40 mg / g TSS, the hydrolysis time was 1 h, and the remaining treatment steps were the same as in Example 1. The results are shown in Table 1: Under the conditions of 40 mg / g TSS enzyme dosage, 1 h hydrolysis time, and 1 mL DES dosage, the sludge cake moisture content could be reduced to approximately 71.72%, and the dewatering resistance could be reduced. The sludge cake moisture content in this example was higher than that in Example 1. The reason for this may be that the hydrolysis time in this example was 1 h, and the EPS was not completely broken down. Not only did it fail to form a loose structure to allow DES to enter the interior and react further with the polysaccharides, but it also hydrolyzed the proteins in the outer EPS, causing the release of some hydrophilic substances and deteriorating the dewatering performance.

[0035] Example 4 (Adjusting the DES molar ratio) The sludge was treated according to the method in Example 1, except that DES-1 was prepared using a choline chloride to citric acid molar ratio of 1:0.8. The remaining treatment steps were the same as in Example 1. The final moisture content of the sludge cake after the reaction was 62.5%, the CST was 21.2 s, and the SRF was 10.12 × 10⁻⁶. 13 The sludge dewatering performance of m / kg was slightly lower than that of Example 1. This may be because the hydrogen bond network of DES-1 is weaker than that of DES (choline chloride:citric acid = 1:1) used in Example 1, resulting in less interaction with polysaccharides and hydrolyzed amino acids, thus leading to poorer dewatering performance than that of Example 1.

[0036] Example 5-19 (Multi-factor experiment) Multi-factor optimization was performed using enzyme dosage, enzyme hydrolysis time, and DES dosage as factors, with cake moisture content as an indicator. In Examples 5-19, except for enzyme dosage, enzyme hydrolysis time, and DES dosage (as shown in Table 1), the remaining treatment steps were the same as in Example 1. Response surface methodology results are shown in [Table 1]. Figure 3 Based on the response surface methodology, after comprehensive optimization of enzyme dosage, hydrolysis time, and DES dosage, the predicted optimal combination is Example 1: 40 mg / g neutral protease TSS, hydrolysis at 40℃ for 2 h, followed by the addition of 1 mL / 100 mL DES for 20 min conditioning. The sludge cake moisture content was obtained through verification experiments.

[0037] (2) Fitting analysis Analysis of variance was performed using the Quadratic model in Design Expert 8.0 software, where Y1 represents the moisture content of the sludge cake, and A, B, and C represent the enzyme dosage, hydrolysis time, and DES dosage, respectively. The fitted polynomial equation is as follows: Y1=60.271+1.6216A-11.572B-18.9595C+0.128AB+0.1025AC+0.545BC-0.023545A 2 +1.0605 B 2 +3.6405 C 2 .

[0038] Figure 3Table 2, together with the data, illustrates the effects of enzyme dosage, DES dosage, and enzyme hydrolysis time on WC%. The model was significant overall (p = 0.0002, F = 24.06); among them, the amount of neutral protease dosage had a significant effect on WC (p = 0.0002). Meanwhile, the mean square of the lack-of-fit term was 0.39, and the lack-of-fit term was not significant (p = 0.72), indicating that the model did not have significant lack of fit and had a good fitting effect, making it suitable for simulating and predicting the dehydration process using neutral protease combined with DES.

[0039] (3) Response surface analysis Based on the analysis results in Table 2, 3D surfaces and contour plots of sludge moisture content after conditioning using combined conditioning technology under different conditions can be obtained, such as... Figure 3 As shown, with the increase of neutral protease dosage, the water content first decreases and then increases. This is mainly because although neutral protease can hydrolyze protein substances in EPS, excessive neutral protease dosage will release a large amount of hydrophilic substances, leading to a final increase in water content. The trend of neutral protease hydrolysis time is similar to that of neutral protease dosage, both showing a trend of first decreasing and then stabilizing. With the increase of hydrolysis time, the water content first decreases and then stabilizes. DES dosage has a secondary effect; 1 mL / 100 mL sludge is optimal, and further increases will increase WC, while the change in hydrolysis time has a smaller impact on WC. According to the ANOVA (analysis of variance) in Table 2 and Figure 3 The model was significant (p<0.05). The optimal process conditions for neutral protease and DES were 40 mg / g TSS of biological enzyme, 1 mL / 100 mL of DES added to sludge, and 2 h of hydrolysis time, with the water content of the sludge cake reaching 60.12%.

[0040] This invention provides a method for the combined treatment of sludge dewatering using biological enzymes and deep eutectic solvents, which has the following advantages: (1) Sequence synergy and avoidance of the adverse effect of hydrophilic rebound after enzymatic hydrolysis: The present invention adopts a stepwise strategy of enzymatic hydrolysis followed by DES. Enzymatic hydrolysis weakens the EPS network and forms permeation channels. DES is introduced within 0–30 min after the end of enzymatic hydrolysis to perform molecular-level modification and hydrophobic reconstruction of the exposed sites. This can effectively suppress the rebound of CST and SRF that may occur under enzyme treatment alone, and achieve simultaneous improvement of solid-liquid separation resistance indicators.

[0041] (2) Under optimal conditions, the moisture content of the mud cake can be reduced from about 77.5% to about 60.12%, and the CST decreases from 25.4 s to 19.2 s, while the SRF decreases from 15.4 × 10⁻⁶. 13 m / kg decreased to 9.55×10 13m / kg. The moisture content of the sludge cake treated with DES alone was approximately 65.17%. This demonstrates that the present invention, under relatively mild conditions and with a lower DES dosage, still achieves a significantly better deep dehydration effect than single treatment.

[0042] (3) Promote floc aggregation and improve mechanical dewatering adaptability: After co-treatment, the sludge zeta potential can be increased from about -15.6 mV to near electrical neutrality (about 0.43 mV), which is conducive to reducing electrostatic repulsion between particles, promoting floc aggregation and structural skeletonization, thereby improving the mechanical dewatering efficiency of pressure filtration, centrifugation or vacuum filtration.

[0043] (4) Good engineering feasibility: The process conditions are mild and the steps are clear, which can be directly embedded into the existing mechanical dehydration process; by limiting the sequence and time window, the dehydration performance can be improved without significantly increasing the consumption of chemicals and viscosity burden, which is convenient for large-scale application.

[0044] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for dewatering sludge using a combination of biological enzymes and a deep eutectic solvent, characterized in that, include: (1) Add biological enzymes to the sludge for hydrolysis to obtain enzymatically hydrolyzed sludge; (2) Add a deep eutectic solvent to the enzymatically hydrolyzed sludge for conditioning, and mechanically dewater the conditioned sludge, wherein the deep eutectic solvent includes choline chloride and organic acids.

2. The method for combined treatment of sludge dewatering using biological enzymes and deep eutectic solvents as described in claim 1, characterized in that, In step (1), the dosage of the bio-enzyme is 20–60 mg / g, based on the total suspended solids in the sludge.

3. The method for combined treatment of sludge dewatering using biological enzymes and deep eutectic solvents as described in claim 1, characterized in that, The bioenzymes include neutral proteases, acidic proteases, and / or alkaline proteases.

4. The method for combined treatment of sludge dewatering using biological enzymes and deep eutectic solvents as described in claim 1, characterized in that, In step (1), the hydrolysis temperature is 35-45℃ and the hydrolysis time is 1-3 hours.

5. The method for combined treatment of sludge dewatering with biological enzymes and deep eutectic solvents as described in claim 1 or 4, characterized in that, During hydrolysis, the stirring speed should be controlled at 80–200 rpm.

6. The method for combined treatment of sludge dewatering using biological enzymes and deep eutectic solvents as described in claim 1, characterized in that, In step (2): a deep eutectic solvent is added to the enzymatically hydrolyzed sludge within 0–30 min after step (1) is completed.

7. The method for combined treatment of sludge dewatering using biological enzymes and deep eutectic solvents as described in claim 1, characterized in that, In step (2), the amount of the deep eutectic solvent added is 1–3 mL / 100 mL of sludge, based on the sludge volume.

8. The method for combined treatment of sludge dewatering with biological enzymes and deep eutectic solvent as described in claim 7, characterized in that, The dosage of the deep eutectic solvent is 1 mL / 100 mL of sludge, based on the volume of sludge.

9. The method for combined treatment of sludge dewatering using biological enzymes and deep eutectic solvents as described in claim 1, characterized in that, The molar ratio of choline chloride to organic acid is 1:0.5–1.

5.

10. The method for combined treatment of sludge dewatering using biological enzymes and deep eutectic solvents as described in claim 1, characterized in that, The mechanical dehydration includes pressure filtration, centrifugation, or vacuum filtration.

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